The present invention relates to a filter with a low-noise amplifier, which is mainly for use in base stations of a mobile communication system such as a vehicular phone and a portable phone.
In recent years, demand for base stations of mobile communication is increasing due to rapid spread of mobile communication. In response to this, there is a keen demand for higher performance and lower cost of low-noise amplifiers used therein. As one example of known filters with low-noise amplifiers, a low-noise amplifier used in a duplexer of a base station is required to have low-noise characteristics and thus, an input matching circuit is designed so as to have a minimum noise figure. On the other hand, this low-noise amplifier is usually used so as to be connected, at its input, to an antenna duplexer. A filter in the antenna duplexer is designed so as to exhibit desired characteristics in a state of impedance matching. Thus, in order to gain the characteristics of the filter in the antenna duplexer, impedance between the filter and the amplifier should be matched. Thus, in order to satisfy both impedance matching and noise figure matching, an isolator is inserted between the filter and the input matching circuit of the low-noise amplifier as described in "National Technical Report" Vol. 32, No. 4, August 1986, pp. 437-438. By employing this circuit configuration, not only impedance matching is obtained at the input terminal but noise figure matching is obtained at the input of the amplifying element.
However, in the above described configuration, the isolator is required to be provided for satisfying both impedance matching and noise figure matching. Since the isolator is inserted at the input side of the low-noise amplifier, insertion loss of the isolator deteriorates noise figure of the circuit directly. In addition, the isolator is quite expensive, which hampers low production cost of the circuit.